Protection circuit and method for floating power transfer device

ABSTRACT

A protection circuit and method are provided for a floating power transfer device having one or more switches for controlling charging of a reservoir capacitor across which a load is applied when in use. The protection circuit includes a control circuit, a fault detection circuit and a precharge driver circuit. The control circuit at least partially controls switching of the at least one switch, while the fault detection circuit detects when a fault in the floating power transfer device or the load occurs and sends a fault detect signal to the control circuit in response thereto. The precharge driver circuit, which is enabled by the control circuit responsive to receipt of the fault detect signal, attempts to precharge the reservoir capacitor to a voltage level sufficient for switching of the one or more switches to proceed without damaging the switches.

REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional No.60/427,633, filed Nov. 18, 2002. This provisional application is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates in general to power transferdevices, and more particularly, to a protection circuit and protectionmethod for low resistance switches of a floating power transfer device.

BACKGROUND OF THE INVENTION

[0003] Many system designs include power conversion circuitry to developa required operating voltage. One such power conversion circuit is knownas a charge pump. A charge pump is a device for creating increases insupply voltage or for inverting a supply voltage to generate a splitsupply. Many of these devices are related to applications usingnon-volatile memory circuits, which require a high voltage forprogramming. In a conventional charge pump power conversion circuit, theload device connects so that one terminal thereof is common to one ofthe supply terminals, typically the ground reference. U.S. Pat. No.4,807,104 discloses a power conversion circuit which is both a voltagemultiplying and inverting charge pump. However, the output of the powerconversion circuit remains referenced to the ground node.

[0004] In certain system implementations, it may be advantageous topower the system using a floating power transfer device. By floating thepower transfer device, if a terminal in the system were to short, thenthe system may still be able to continue to operate. For example, in anautomobile bus network, the signaling portion of the system on the buscould be floating relative to any other reference, such as ground orV_(dd). This would provide enhanced fault tolerance by allowingcommunications to still occur notwithstanding a short at a terminalthereof.

SUMMARY OF THE INVENTION

[0005] The shortcomings of the prior art are overcome and additionaladvantages are provided through the provision of a protection circuitfor a floating power transfer device. The protection circuit includes acontrol circuit, a fault detection circuit and a precharge drivercircuit. The control circuit controls switching of at least one switchof the floating power transfer device, where the at least one switchcontrols charging of a reservoir capacitor of the device across which aload is applied when in use. The fault detection circuit detects when afault occurs in at least one of the floating power transfer device orthe load, and sends a fault detect signal to the control circuitresponsive thereto. The precharge driver circuit precharges thereservoir capacitor and is enabled by the control circuit responsive toreceipt of the fault detect signal from the fault detection circuit.When enabled, the precharge driver circuit attempts to precharge thereservoir capacitor to a voltage level sufficient for switching of theat least one switch to proceed without damaging the switch.

[0006] In another aspect, a floating power transfer device is provided.The floating power transfer device includes a reservoir capacitor acrosswhich a load is applied when in use and a power supply voltage forcharging the reservoir capacitor. At least one switch is coupled betweenthe power supply voltage and the reservoir capacitor to selectivelyconnect and disconnect the power supply voltage from the reservoircapacitor. A protection circuit is provided for the at least one switch.This protection circuit includes a control circuit, a fault detectioncircuit and a precharge driver circuit. The control circuit at leastpartially controls switching of the at least one switch of the floatingpower transfer device, while the fault detection circuit detects a faultin either the floating power transfer device or the load, and responsivethereto sends a fault detect signal to the control circuit. Theprecharge driver circuit is enabled by the control circuit responsive toreceipt of the fault detect signal, and when enabled, attempts toprecharge the capacitor to a voltage level sufficient for switching ofthe at least one switch to proceed without damaging the at least oneswitch.

[0007] In a further aspect, a method for protecting switches of afloating power transfer device is provided. This method includes:controlling switching of at least one switch, the at least one switchcontrolling charging of a reservoir capacitor of the floating powertransfer device across which a load is applied when in use; monitoringat least one of the floating power device and the load for detecting afault, and upon detecting a fault, generating a fault detect signal; andresponsive to generating of the fault detect signal, attempting toprecharge the reservoir capacitor to a voltage level sufficient forswitching of the at least one switch to proceed without damaging the atleast one switch.

[0008] Additional features and advantages are realized through thetechniques of the present invention. Other embodiments and aspects ofthe invention are described in detail herein and are considered a partof the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The subject matter which is regarded as the invention isparticularly pointed out and distinctly claimed in the claims at theconclusion of the specification. The foregoing and other objects,features, and advantages of the invention are apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings in which:

[0010]FIG. 1 is a schematic of one embodiment of a ground referenced,power transfer device for supplying power to a load via a groundreferenced capacitance;

[0011]FIG. 2 is a schematic of one embodiment of a floating powertransfer device for delivering power to a load through a floatingreservoir capacitance;

[0012]FIG. 3 is a schematic of one embodiment of a protection circuitfor a floating power transfer device, in accordance with an aspect ofthe present invention;

[0013]FIG. 4 is a schematic of another embodiment of a protectioncircuit for a floating power transfer device, in accordance with anaspect of the present invention;

[0014]FIG. 5 is a schematic of still another embodiment of a protectioncircuit for a floating power transfer device, in accordance with anaspect of the present invention;

[0015]FIG. 6 is a schematic of one embodiment of a fault detectioncircuit for use in the protection circuit of FIGS. 3-5, in accordancewith an aspect of the present invention;

[0016]FIG. 7 is a schematic of one embodiment of a comparator with agate-clamp referenced to the ground node for use in the fault detectioncircuit of FIG. 6, in accordance with an aspect of the presentinvention;

[0017]FIG. 8 is a schematic of another embodiment of a comparator with agate-clamp referenced to a positive supply for use in the faultdetection circuit of FIG. 6, in accordance with an aspect of the presentinvention; and

[0018]FIG. 9 is a detailed schematic of one embodiment of a prechargedriver circuit for the protection circuits of FIGS. 3-5, in accordancewith an aspect of the present invention.

DESCRIPTION OF EMBODIMENTS

[0019] Reference is now made to the drawings, wherein the same referencenumbers used throughout different figures designate the same or similarcomponents. One embodiment of a power transfer device for powering aload 12 is show in FIG. 1. This charge transfer device delivers chargeonto a capacitor 11 through a switch 13, under control of a signalgenerator 15. Charge is provided by a power supply voltage (V_(dd)) 14.The power transfer device of FIG. 1 is referred to as a groundreferenced charge transfer device since the device supplies power to theload via a ground referenced capacitance.

[0020] A floating version of a power transfer device is depicted in FIG.2. In this figure, a load 21 is powered by a reservoir capacitance 22.Charge is delivered onto capacitor 22 through switches 23 and 24, whichare operated in tandem under control of a voltage switch signal 25. Whenboth switches are closed (i.e., turned on), the power supply voltage(V_(dd)) 26 is applied across the capacitor 22. This power transferdevice is floating because capacitor 22 is isolated from the groundedpower supply 26 when the charging switches 23, 24 are opened (i.e.,turned off). When the circuit is used to deliver power into a loaddevice (e.g., as part of a voltage doubler), it is expected that therewill be a permanent bias voltage across the capacitor. The powerdelivered to the load creates a ripple voltage on the reservoircapacitor, superimposed onto the bias voltage. When switches 23, 24 areclosed (i.e., turned on), the lost charge (Q) is replenished and thereservoir capacitor voltage increases to the source value (V_(dd)).

[0021] The load in FIGS. 1 or 2 may be of any type of circuit, includinga replication of either circuit shown, with the capacitor (11 or 22)replacing the voltage source (14 or 26), respectively. Inmetal-oxide-semiconductor (MOS) integrated circuits, the switch isimplemented by a transistor. This device presents an on-resistance thatdetermines the dynamics of the circuit operation.

[0022] When the power transfer device is initially turned-on, or thereis a shorting fault across the capacitor (or the load), the full supplyvoltage is applied across the switch devices. Normally these circuitsare used in low power applications where the switch resistance may bequite high and the supply voltage is generally low (e.g., less than 5V).In such a case, it may not be necessary to protect against suchoperating conditions.

[0023] However, start-up and fault conditions create a potentiallydamaging operating state if the charge transfer device is used todeliver power to the load. For such a device, switch transistors aremade low ohmic to reduce system losses, which also diminishes powerlosses during the charging phase of operation. When there is nopreexisting bias present on the reservoir capacitor (e.g., capacitor 11or capacitor 22 in FIG. 1 or 2), it is possible for very high currentsto flow through the switch. For example, with a 1Ω switch resistance,and a 20V supply, a current of 20A is possible, briefly dissipating400W. With discrete devices, this may be possible, but not with low costintegrated solutions. Normally, the circuit might present a 1Vdifference across the switch, resulting in a more manageable current of1A.

[0024] Currently, floating charge transfer devices concentrate on lowpower systems that can absorb the increase in power during start-up. Inthese systems, the switches are generally of higher impedance than inthe case of a floating power transfer device such as discussed herein.

[0025] Thus, provided herein is a protection circuit and protectionmethod to prevent excessive currents and power dissipation during, forexample, start-up or fault conditions, in floating capacitor chargecircuits, referred to herein as floating power transfer devices. Theprotection circuit described below is able to directly or indirectlydetect, for example, a low voltage across the reservoir capacitor duringeither phase of operation. One characteristic of the floatingcapacitance is the ability of the capacitor to float above or below thepower supply ground reference during the period that the switches aredisabled (i.e., turned off). Also, one issue to be addressed inproviding a protection circuit for the switches is that the detection ofa fault needs to be communicated from the floating capacitor side of thepower transfer device (i.e., nodes Cap+, Cap−) to the grounded supplyside (i.e., nodes V_(dd), gnd).

[0026] One embodiment of a floating power transfer device and protectioncircuitry, in accordance with an aspect of the present invention, isdepicted in FIG. 3. The device includes a pair of switches 36, 37, usedduring normal operation, a reservoir capacitor 35, and a power source 49connected at one end to ground 48. These components, taken together,form a circuit similar to that depicted in FIG. 2. This circuit suppliespower to a load 50 when in use.

[0027] The protection circuitry includes a fault detection circuit 33,which can directly or indirectly monitor voltage across capacitor 35. Inthis example, fault detection circuit 33 is connected between terminalsCap+ 31 & Cap− 32, possibly deriving its power supply from the sameterminals. The fault detection circuit may be a passive detector that iscapable of operating over all possible voltages, from 0V up to anarbitrary maximum. A fault is determined to occur, in one example, whenvoltage across reservoir capacitor 35 falls below a fault threshold.This threshold is set low enough to allow normal operation, while highenough to prevent damage from occurring due to excessive currentsflowing through the switches 36, 37. For instance, with a 20V supply,and a switch resistance of 1Ω, a maximum current of 2A would set aminimum capacitor voltage of 16V before protection is required. So, if ashort occurs during normal operation, or some other event causes thecapacitor voltage to fall below the fault threshold (e.g., 16V in thisexample), then a fault detect signal 46 is asserted. This signal istransferred to a control circuit 43 through a floating-to-ground shifter34 as output 45 from the floating level shift circuit. Circuit 34connects between floating nodes 31, 32, as well as between the groundreferenced nodes 41, 40. Control circuit 43 may be implemented as alogic circuit, or as a program which processes the fault detect signaland decides whether to allow the main switches 36 & 37 to turn-on.

[0028] At turn-on, there is a voltage available from source 49, butreservoir capacitor 35 is completely discharged, i.e., the capacitorvoltage is 0V. In this case, a fault is detected by the fault detectioncircuit 33 and its presence is signaled to control circuit 43. To enableswitches 36, 37 while the capacitor remains in this state would lead tothe failure of the switches. This might be an immediate failure, or itmay manifest itself as a curtailed lifetime for the components,depending upon the time taken to restore the capacitor's voltage to itsnormal state.

[0029] In one implementation, the control circuit 43 serves as aninterface between the normal control logic and switches 36, 37. Controlsignals from an external device determine the switch state (on nodeswitch 44), through interface node uPIO 42. The fault_IN connectsthrough the float level shift circuit 34 to the fault detection circuit33. Additional signals indicating a fault state may be made available tothe external device through interface uPIO 42.

[0030] When a fault is asserted, the control block 43 insures that theswitches 36, 37 are disabled, preventing further dissipation by theseswitches. On the next appropriate control phase (i.e., when the switcheswould normally be enabled), a separate precharge driver circuit 47 isenabled. This circuit 47 is capable of delivering charge to thereservoir capacitor 35 without causing damage to the circuit. Itachieves this by using current-limited output devices that prevent thecharging process from causing excessive power dissipation. When theswitch on (SWON) input to recharge driver circuit 47 is enabled, outputs38, 39 turn-on and the capacitor charges. The control circuit 43 mayenable these outputs 38, 39 continuously until the detected faultcondition is removed, or it may cycle through charging and hold phases,emulating the normal mode of operation. By this method, the protectioncircuit prevents damaging currents from flowing through the powertransfer device during the start-up phase. One consequence of thistechnique is the requirement for a minimum start-up period before normaloperation is commenced. The duration of this period is determined by thevarious factors affecting the circuit operation and the level ofprotection required. In a practical implementation, an additional delayof several normal switch cycles may be added to insure that the systemhas reached a stable operating state before enabling the completecircuit. An external control device may be aware of the start-upcondition and use that information to enable the start-up sequencedescribed above. In such a case, it is possible to use different controlsequences for start-up and fault conditions.

[0031] When a fault occurs during normal operation that causes, forexample, the voltage on capacitor 35 to fall below the set faultthreshold, then switches 36, 37 are turned off and the control block 43attempts to restart the circuit. This may follow the full start-up cycle(when there is no distinction between start-up and fault), or it mayfollow a shortened cycle. A shortened cycle would charge the capacitor35, then turn-off the precharge driver circuit 47 and evaluate the faultsignal again. If no further fault state is detected, then control isreturned to normal operation. With the full cycle, a repeated start-upattempt is made. When a predetermined number of attempts is exceeded,the circuit is resolved to be in a fault state and the control circuit43 disables further attempts until reset by some external control. Afault state signal can be passed back to the external control throughthe uPIO node 42.

[0032]FIG. 4 depicts a refinement of the protected, floating powertransfer device of FIG. 3. In this embodiment, a thermal detectioncircuit 60 is provided to complement the fault detection. Temperaturedetection can be useful when the device and detection circuit areimplemented on a single integrated circuit (IC). When a fault occurs andthe circuit operates close to the fault detection threshold, the IC mayshow excessive power dissipation. The chip temperature increases rapidlywhen this happens and the thermal detection circuit 60 can detect atemperature change beyond the normal expected operating range. Theoutput signal 61 asserts a fault to the control circuit 43. In thiscase, the type of fault can be registered and output switches 36, 37disabled. A restart can be attempted and if the fault is detected again,the control circuitry can flag the fault back to the external logic,again, by the uPIO 42 interface. When this happens, the control circuitprevents any further operation of the switches until an external signalinitiates a restart. The balance of the protection circuit and floatingpower transfer device depicted in FIG. 4 operates as described above inconnection with FIG. 3.

[0033] In certain circumstances, the protection circuitry of FIGS. 3 & 4may be simplified by eliminating the float level shift circuit 34 ofthose figures. The resulting implementation is depicted in FIG. 5. Byeliminating the float level shift circuit, fault detection becomespossible only while switches 36, 37 are enabled. The output of the faultdetection circuit 43 is also now referenced to the ground node 48. Theoperating principal of this variation has the fault detection occurringduring the first part of the switch closure phase. Alternatively, thelow-current precharge driver circuit 47 can be enabled prior to theswitches, allowing a short detection phase to occur ahead of theprincipal power phase, which enables the switches 36, 37. When thismethod is employed, it is possible for the detection circuit connectedbetween the capacitors to be mainly passive devices.

[0034] Various specific details of implementation of the protectioncircuit embodiments of FIGS. 3-5 are described below with reference toFIGS. 6-9. The protection circuit described herein can form part of acustom IC that uses double-diffused metal-oxide semiconductor (DMOS)transistors for the switching devices. A silicon-on-insulator (SOI)structure allows individual transistors to be electrically isolated fromone another. The fault detection could follow the form shown in FIG. 5,using passive detection devices.

[0035]FIG. 6 depicts one embodiment of a fault detection circuit 33.This circuit includes a fault reference generating circuit 70, producingtwo output reference values 78, 79 (i.e., ref− 78 relative to ground,and ref+ 79 relative to the positive supply 49). Two comparators 71 & 72compare these reference voltages to the voltages on the reservoircapacitor 35 during the period that the switches are closed (i.e.,turned on). This allows the detection of excess voltage drops acrossindividual switches 36, 37. The reason for this is to enable thedetection of shorts on the capacitor output nodes 31, 32. A short mayonly show a fault at one terminal, so both should be tested to insurefull coverage of potentially damaging fault conditions. The resultingoutputs may be combined into a single fault detect signal or flag 46.The comparators have high impedance inputs and gate clamps to protecttheir inputs from damage during the floating phase of operation. Thefault reference generating circuit 70 may be any circuit capable ofgenerating voltage references relative to both supplies. The valuesrequired for the reference voltages are determined by the switchresistance and the ability of the IC to dissipate power.

[0036] Other embodiments of the comparator circuitry 72, 71 for thefault detection circuit 33 of FIG. 6 are depicted in FIGS. 7 & 8,respectively. A comparator with a gate-clamp referenced to the groundnode is depicted in FIG. 7, while a positive supply referencedgate-clamp is shown in FIG. 8. The operating principle is the same forboth. The high value series input resistor R 84, 83 isolates the inputfrom the comparator 72, 71 of FIGS. 7 & 8, respectively. A zener diode86, 96 clamps the comparator input to a voltage that does not exceed itsinput breakdown voltage. If the input swings beyond the supply voltage(ground or V_(dd) in FIGS. 7 & 8, respectively), then the zener diode86, 96 acts like a diode and limits the input swing in the oppositedirection.

[0037] The precharge driver circuit 47 in FIGS. 3-5 may be any circuitthat can be controlled, and is capable of limiting the current deliveredto the capacitor reservoir 35. In one implementation, the prechargedriver circuit can act as a current source on both output terminals. Inthe absence of a fault, the reservoir capacitor charges at apredetermined rate. As the capacitor charges, the current-sourcebehavior changes from current source to a high-impedance switch. Theactual switch impedance is set to allow the start-up time to beminimized, without introducing damaging power dissipation.

[0038] The circuit of FIG. 9 shows one implementation of the prechargedriver circuit 47, which includes a current reference sub-block and anoutput section. A current source 101 defines a low-value current (e.g.,2 μA). Transistors 102, 103, 104 form a current mirror with two outputs,including node SRC− 126, that reflect the reference current. The outputfrom transistor M_1 103 is used in the complementary current mirrorformed by transistors 105, 106, creating a single output node SRC+ 127.Two switches 108, 109 controlled by the ENABLE input 107 disable theoutputs SRC+, SRC− of the current reference sub-block. The complementaryoutput sections 112, 113, 121, 122, 123, 124, 125 and 114, 115, 116,117, 118, 119, 120 create current-limited outputs controlled by theENABLE input. When switches 120, 121 are on, the outputs are off and nocurrent is available at the terminals SW+ and SW− 125, 119. The outputdiodes 118, 124 insure that the implicit diodes of the output DMOStransistors 116, 123 are never forward biased when the reservoircapacitor (connected between the SW+ and SW− terminals 125, 119), isfloating relative to ground 111. Current scaling is used to boost thereference current by a factor of 10× in the current reference sub-block.A further 200× is obtained by a combination of resistor 112, 113 ratioand transistor ratio 122, 123, for output SW+ 125. Output SW− 119 isscaled in a similar manner.

[0039] The overall accuracy obtained by the precharge driver circuit isnot critical to its performance. Its primary function is to enable thesafe charging of the reservoir capacitor after the circuit is started ora fault is detected. The timing of the start-up may be improved bytighter control of the charging currents, but the benefit has to beweighed against increased circuit complexity. In normal operation, theprecharge driver circuit 47 may switch only during the controlledstart-up, or it may switch continuously in synchronism with the mainswitches 36, 37.

[0040] The control circuit 43 (see FIGS. 3-5) contains the digitalfunctionality for the system. This control circuit receives a faultdetect signal, responds thereto by turning off the switches 36, 37 andreport backs to an external device. The external device may initiate thestart-up (assuming an a-priori knowledge of the system) through thestart_control circuit 43.

[0041] Although preferred embodiments have been depicted and describedin detail herein, it will be apparent to those skilled in the relevantart that various modifications, additions, substitutions and the likecan be made without departing from the spirit of the invention and theseare therefore considered to be within the scope of the invention asdefined in the following claims.

What is claimed is:
 1. A protection circuit comprising: a controlcircuit for controlling switching of at least one switch of a floatingpower transfer device, the at least one switch controlling charging of areservoir capacitor of the floating power transfer device across which aload is applied when in use; a fault detection circuit for detecting afault in at least one of the floating power transfer device or the load,and for sending a fault detect signal to the control circuit responsivethereto; and a precharge driver circuit for pre-charging the reservoircapacitor, the precharge driver circuit being enabled by the controlcircuit responsive to receipt of the fault detect signal from the faultdetection circuit, wherein when enabled, the precharge driver circuitattempts to precharge the reservoir capacitor to a voltage levelsufficient for switching of the at least one switch to proceed withoutdamaging the at least one switch.
 2. The protection circuit of claim 1,wherein the fault detection circuit resides in a floating portion of thefloating power transfer device and the control circuit resides in aground referenced portion of the floating power transfer device, andwherein the protection circuit further comprises a float level shiftcircuit for shifting the fault detect signal from the floating portionof the floating power transfer device to the ground referenced portionfor forwarding to the control circuit.
 3. The protection circuit ofclaim 1, wherein the fault detection circuit further comprises circuitryfor directly or indirectly monitoring when voltage across the reservoircapacitor of the floating power transfer device falls below a faultthreshold, and for sending the fault detect signal to the controlcircuit responsive thereto.
 4. The protection circuit of claim 1,wherein the floating power transfer device further comprises a powersupply having a voltage level in a range of 5 to 20 volts, the powersupply charging the reservoir capacitor of the floating power transferdevice when the at least one switch is turned on.
 5. The protectioncircuit of claim 4, wherein the at least one switch comprises twoswitches operated in tandem for cyclically applying the power supplyvoltage across the reservoir capacitor to charge the capacitor.
 6. Theprotection circuit of claim 1, further comprising a temperature sensorfor detecting when temperature of the at least one switch rises above aset temperature level, and for sending an over temperature signal to thecontrol circuit responsive thereto, and wherein the control circuitfurther comprises means for temporarily shutting down the floating powertransfer device and subsequently reinitiating a startup procedureresponsive to receipt of the over temperature signal.
 7. A devicecomprising: a reservoir capacitor across which a load is applied when inuse; a power supply voltage for charging the reservoir capacitor; atleast one switch coupled between the power supply voltage and thereservoir capacitor to selectively connect and disconnect the powersupply voltage from the reservoir capacitor; and a protection circuitfor the at least one switch, the protection circuit including: a controlcircuit for controlling switching of the at least one switch of thedevice, a fault detection circuit for detecting a fault in at least oneof the device or the load, and for sending a fault detect signal to thecontrol circuit responsive thereto, and a precharge driver circuit forprecharging the reservoir capacitor, the precharge driver circuit beingenabled by the control circuit responsive to receipt of the fault detectsignal from the fault detection circuit, and wherein when enabled, thepreharge driver circuit attempts to precharge the reservoir capacitor toa voltage level sufficient for switching of the at least one switch toproceed without damaging the at least one switch.
 8. The device of claim7, wherein the fault detection circuit resides in a floating portion ofthe floating power transfer device and the control circuit resides in aground referenced portion of the floating power transfer device, andwherein the protection circuit further comprises a float level shiftcircuit for shifting the fault detect signal from the floating portionof the floating power transfer device to the ground referenced portionfor forwarding to the control circuit.
 9. The device of claim 7, whereinthe fault detection circuit further comprises circuitry for directly orindirectly monitoring when voltage across the reservoir capacitor fallsbelow a fault threshold, and for sending the fault detect signal to thecontrol circuit responsive thereto.
 10. The device of claim 7, whereinthe power supply has a voltage level in a range of approximately 5 toapproximately 20 volts and the power supply charges the reservoircapacitor when the at least one switch is turned on.
 11. The device ofclaim 10, wherein the at least switch comprises two switches operated intandem for cyclically applying the power supply voltage across thereservoir capacitor to charge the capacitor.
 12. The device of claim 7,wherein the protection circuit further comprises a temperature sensorfor detecting when temperature of the at least one switch rises above aset temperature level, and for sending an over temperature signal to thecontrol circuit responsive thereto, and wherein the control circuitfurther comprises means for temporarily shutting down the floating powertransfer device and subsequently reinitiating a start-up procedureresponsive to receipt of the over temperature signal.
 13. A methodcomprising: controlling switching of at least one switch, the at leastone switch controlling charging of a reservoir capacitor of a floatingpower transfer device across which a load is applied when in use;monitoring at least one of the floating power device and the load fordetecting a fault, and upon detecting a fault, generating a fault detectsignal; and responsive to generating of the fault detect signal,attempting to precharge the reservoir capacitor to a voltage levelsufficient for switching of the at least one switch to proceed withoutdamaging the at least one switch.
 14. The method of claim 13, whereinthe controlling is performed from a ground referenced portion of thefloating power transfer device and the monitoring is performed from afloating portion of the floating power transfer device, and wherein themethod further comprises level shifting the generated fault detectsignal from the floating portion to the ground referenced portion foruse in initiating the attempting to precharge the reservoir capacitor.15. The method of claim 13, wherein the monitoring comprises monitoringdirectly or indirectly when voltage across the reservoir capacitor fallsbelow a fault threshold, and for generating the fault detect signalresponsive thereto.
 16. The method of claim 13, wherein charging of thereservoir capacitor of the floating power transfer device is from apower supply having voltage level in a range of 5 to 20 volts, whereinthe power supply charges the reservoir capacitor of the floating powertransfer device when the at least one switch is turned on.
 17. Themethod of claim 16, wherein the at least one switch comprises twoswitches operated in tandem for cyclically applying the power supplyvoltage across the reservoir capacitor to charge the capacitor.
 18. Themethod of claim 13, further comprising monitoring temperature of the atleast one switch, and generating an over temperature signal when thetemperature of the at least one switch rises above a set temperaturelevel, and wherein the method further comprises temporarily shuttingdown the floating power transfer device and subsequently reinitiating astart-up procedure responsive to generating of the over temperaturesignal.
 19. A circuit comprising: means for controlling switching of atleast one switch, the at least one switch controlling charging of areservoir capacitor of a floating power transfer device across which aload is applied when in use; means for monitoring at least one of thefloating power device and the load for detecting a fault, and upondetecting a fault, for generating a fault detect signal; and means forattempting, responsive to generating of the fault detect signal, toprecharge the reservoir capacitor to a voltage level sufficient forswitching of the at least one switch to proceed without damaging the atleast one switch.